Tri-lobe flowmeter rotor with partially open core structure

ABSTRACT

There is a gear rotor for use in a positive displacement flowmeter of the type described. The rotor has an external wall profile  8  forming three equally spaced lobes  9  and a partially open core structure inside said profile with internal cavities  15, 15 A defined between webs  13, 14  extending from a central hub  12  of said rotor to said wall profile  8.

FIELD OF THE INVENTION

This invention relates to apparatus for fluid flow measurement. More particularly, although not exclusively, it discloses improvements in rotors for positive displacement flowmeters.

BACKGROUND TO THE INVENTION

Positive displacement flowmeters are known which, as shown schematically in FIG. 1, comprise a pair of meshed tri-lobe rotors 1 formed with gear teeth 2 and rotatable on spaced apart parallel axis 3 within a housing 4. The rotors are rotatably offset by 60 degrees and are meshed together in a critical relationship which causes said rotors to rotate in the direction of arrows 5 as fluid passes through from an inlet chamber 6 to an outlet chamber 7. The speed of rotation of the rotors is related to and indicative of the fluid volume flow through the meter per unit of time. Each rotor passes three displacements of fluid per revolution. The three lobe configuration of the rotors provides significant advantages over oval two lobe rotors in terms of vibration, backlash, noise and fluid bypass reduction. In order to provide further advantages over prior art metal rotors it has also been proposed to injection mould tri-lobe rotors from an engineering plastic such as RYTON or similar. The principal advantages of plastic rotors include the following:

-   -   Lower cost to cast     -   Reduced machining cost     -   High chemical resistance     -   Broader application due to chemical resistance     -   Greater accuracy due to lower inertia     -   Greater turn down ratios     -   Reduced drive energy

It has been found by the inventors however that the currently proposed solid core plastic tri-lobe rotors are difficult to produce with known injection moulding techniques and post treatments. This is because the critical tooth profiles and mating surfaces are compromised during cooling by distortion from surface sinking, shrinkage and stresses caused during the moulding process.

It is therefore an object of this invention to ameliorate the aforementioned disadvantages.

It is a further object to provide a tri-lobe rotor of reduced operating weight and inertia compared to solid core plastic rotors.

Accordingly a gear rotor of engineering plastic is disclosed for use in a positive displacement flowmeter of the type described, said rotor having an external wall profile forming three equally spaced lobes and a partially open core structure inside said profile with internal cavities defined between webs extending from a central hub of said rotor to said wall profile.

Preferably the external wall profile between said lobes is concave.

It is further preferred that said engineering plastic is RYTON or similar engineering GRP to ensure stability of casting.

It is further preferred that at least some of said webs extend between said hub and the crests of the lobes.

It is further preferred that others of said webs extend between said hub and the roots of the profile concavities between the lobes.

In another form of the invention the rotor may be formed from metal with a similar partially open core structure in order to reduce weight for improved flow of the flowmeter.

BRIEF DESCRIPTION OF THE DRAWINGS

One currently preferred embodiment of the invention will now be described with reference to the attached drawings in which:

FIG. 1 is a schematic drawing of an end view of a positive displacement flowmeter;

FIG. 2 shows a perspective view of a flowmeter gear rotor according to said invention from one end;

FIG. 3 shows an elevated end view of the flowmeter gear rotor of FIG. 2;

FIG. 4 shows a perspective view of the gear rotor of FIGS. 2 and 3 from the opposite end; and

FIG. 5 shows an elevated end view of the gear rotor of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The gear rotor may comprise an external wall profile 8 forming three equally spaced lobes 9. Between adjacent lobes the profile is concaved at 10. The wall profile B has precisely shaped tooth profiles 11 with critical mating surfaces in order to mesh with another like rotor to perform volumetric fluid flow measurement as described earlier. Such tooth and surface profiles will not be further discussed here in detail as they are well known to persons skilled in the art of flowmeter design and manufacture.

In order to achieve the required stability for the tooth profiles and mating surfaces when using known injection moulding techniques and post treatments the inventors have found that a non-solid core structure for the rotor is advantageous. Therefore in accordance with the invention the core area of the rotor may be moulded with a central hub 12 for rotation on an axis (not shown) and a plurality of structural webs 13, 14 extending outwardly from said hub to support the external wall profile 8. Pairs of internal cavities 15 and L5A are defined at symmetrical positions within the core by these webs which run the length of the rotor. With this embodiment there are six webs. Some of the webs 13 preferably extend between the hub 12 and the crests of the lobes.

Others of the webs 14 preferably extend between the hub 12 and the roots 17 of the profile concavities 10. This currently preferred arrangement for the webs has been found to enable a significant reduction in the mass of the rotor core while still providing a structurally robust support for the wall profile 8. It is also currently preferred that the dimensions of the webs and hub section of the rotor core be chosen such that the wall thickness remains similar throughout the casting. This ensures that the cooling time remains constant for all sections of the gear rotor moulding. The wall thickness varies with the size and mass of the casting and sizes may vary by those skilled in the art. This enables sufficient material flow during the injection moulding process but still avoids profile distortion from shrinkage, propping and stress buildup in the plastic during subsequent cooling and post treatment of the rotor.

As best shown in FIGS. 2 and 3 magnets 16 are preferably implanted in one end of the rotor at the junctions of the webs and wall profile 8. These activate an external reed switch, hall effect device or other means (not shown) to generate a signal related to rotor speed and thus volume flow through the meter.

It will thus be appreciated that this invention at least in the form of the embodiment disclosed discloses a novel and improved core structure for plastic tri-lobe flowmeter rotors. The cavitied core structure provides a structurally robust and serviceable plastic rotor which is able to be injection moulded to the required critical shapes for the tooth profiles and mating surfaces without the distortion arising with prior art solid cores. Clearly however the example described in only the currently preferred form of the invention and a variety of modifications may be made which would be apparent to a person skilled in the art.

For example the number, shape, configuration and placement of the webs is not limited to the current embodiment and may be changed following further research and development work by the inventors. The invention is also limited to any specific plastic although an engineering plastic such as Ryton is currently preferred. 

1. A gear rotor of injection moulded engineering plastic for use in a positive displacement flowmeter of the type described, said rotor including a central hub for rotation on an axis, an external wall profile forming three equally spaced lobes arranged symmetrically around said central hub and a partially open core structure inside said wall profile with internal cavities defined between support webs extending between said central hub of the rotor and said wall profile.
 2. The gear rotor as claimed in claim 1 wherein portions of said external wall profile between said lobes are concave.
 3. The gear rotor as claimed in claim 2 wherein at least some of said webs extend between said hub and crests of the lobes and run the length of the rotor.
 4. The gear rotor as claimed in claim 3 wherein others of said webs extend between said central hub and the roots of the profile concavities and run the length of the rotor.
 5. The gear rotor as claimed in claim 4 wherein pairs of said internal cavities are defined symmetrically within said core structure.
 6. The gear rotor as claimed in claim 5 wherein the dimensions of the webs and central hub are chosen such that wall thicknesses remain substantially constant throughout the rotor
 7. The gear rotor as claimed in claim 6 wherein magnets are implanted in one end of the rotor at junctions of the webs and external wall profile.
 8. The gear rotor as claimed in claim 7 wherein said engineering plastic is Ryton. 